weathering properties of wood species treated with

PEER-REVIEWED ARTICLE bioresources.com

Ozgenc et al. (2012). “Wood coatings & weathering,” BioResources 7(4), 4875-4888. 4875

WEATHERING PROPERTIES OF WOOD SPECIES TREATED WITH DIFFERENT COATING APPLICATIONS

Ozlem Ozgenc,

a,* Salim Hiziroglu,

b and Umit C. Yildiz

c

The objective of this study was to evaluate the discoloration of European beech (Fagus sylvatica) and Scots pine (Pinus sylvestris) specimens treated with different chemicals and surface coated with different UV absorbers before being subjected to artificial weathering. The results showed that the influence of coatings containing UV absorbers (UV screeners micronized TiO2 and UVA of hydroxyphenyl-s-triazine types) were similar to each other. The UV screener TiO2 led to the least discoloration of the coated wood surface, closely followed by the UVA of hydroxyphenyl-s-triazines (HPT). The color stability was determined to be better for pine wood treated with micronized copper preservative coated with UV absorber, in comparison to when it was only coated with UV absorbers and then subjected to weathering. Microscopic observation revealed that the clear-coats penetration behavior was different in wood preservative-treated and in untreated wood of Scots pine, which has various extractives. However, the color stability and coating penetration was nearly the same in beech wood treated with preservatives and in untreated beech wood. We provide an explanation for why these effects occurred and discuss the implications of our findings for the development of weather-resistant wood materials.

Keywords: Accelerated weathering; Wood Preservative; Coating; Color change

Contact information: a: Faculty of Technology, Department of Woodworking Industry Engineering,

Karadeniz Technical University 61830 Trabzon –Turkey; b: Department of Natural Resource, Ecology &

Management, Oklahoma State University, Stillwater, Oklahoma 74078-6013-USA; c: Faculty of Forestry,

Department of Forest Products Engineering, Karadeniz Technical University, 61080 / Trabzon-Turkey;

*Corresponding author: [email protected]

INTRODUCTION

Weathering is a complex process that modifies and degrades the overall molecular

structure of wood and wood-based products. Generally, surface degradation of wood due

to weathering is initiated by solar radiation, as well as other environmental factors. It is a

well known fact that the shorter wavelengths of the solar spectrum having higher photon

energies are more deeply absorbed into polymeric materials such as wood-based material,

and the structure of chemical bonding within the cell wall is adversely influenced. The

weathering initially causes the color of the wood surface to change, followed by the

occurrence of surface checking and increased roughness of the samples. The discoloration

of the surface is a direct indication of the chemical modification in the cell wall due to

weathering. Most of the time, weathered wood would have a more pronounced yellow

color than unexposed wood as a result of the modification of lignin and hemicelluloses.

Such discoloration in the cell wall is influenced by photochemical reactions leading to the

degradation of wood constituents, mainly lignin. A major part of solubilized lignin during

degradation is washed out by rain. However, fiber-rich cellulose with a higher resistance

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against ultraviolet light degradation remains in the cell wall without significant

modification and results in the wood acquiring a whitish to gray color (Feist and Hon

1984; Rowell and Barbour 1988).

Wood products used under outdoor environmental conditions are treated or

finished using different types of chemicals with the purpose of protection so that the

overall service life of the product is extended. Two basic types of finishing or treatment

methods are commonly used to enhance resistance of wood surfaces against weathering.

The first one is the application of finishes to the surface in the form of thin layers or

coatings with limited penetration. Numerous methods including pressurized and non-

pressurized applications of chemicals, such as arsenic-based and others are also widely

employed to protect wood against weathering. In certain applications, both treatment and

finishing can also be used for enhanced weathering resistance of wood (Williams 2005;

Jacques 2000).

Several research projects have investigated the application of inorganic chemicals

to the surface of treated wood, including copper ethanolamine (Cu-MEA) to enhance its

resistance against weathering (Zhang et al. 2009). It was determined that a Cu-MEA

treatment delays the degradation of lignin due to weathering. It was also found that the

discoloration of wood decreased with increasing concentration of copper in the chemical

(Williams 2005). According to findings of a past study, the photo-stabilization of

pressure-treated wood using chromated copper arsenate (CCA), ammonium copper quat

(ACQ1900), and linseed oil also decreased delignification, resulting in a reduction in

wood discoloration (Temiz et al. 2007). The treatment of solid wood with a melamine-

based resin was considered for possible protection of wood products against weathering

without changing their natural appearance (Hansmann et al. 2006). However, it has some

limitation due to its high cost. Another method is the application of clear-coats, which is

the easiest and most common method for protecting wood against weathering

degradation. Usually coating wood with water-borne transparent acrylic finishes is not

very effective against photo-discoloration. However, wood products coated with the

semitransparent acrylic was found effective against photo-degradation, which might be

due to having a pigment content restricting transmittance of UV light to the wood surface

(Schaller and Rogez 2007; Allen et al. 2002). Therefore, it is important to reduce the light

energy reaching the wood surface with use of a coating to prevent its photo-degradation

(Deka and Petrič 2008).

In the last decade, organic and inorganic UV absorbers have received great

attention in transparent wood coatings because of their excellent properties as UV

blockers. It is known that there is no significant difference between acrylic clear-coats

containing the organic and inorganic UV absorbers for stabilizing wood color and

protecting the quality of the surface. However, after the exposure of the coated wood to

weathering, the color stabilization and quality of both the acrylic clear-coat surfaces

showed better quality than that of the control samples (Özgenç et al. 2010). The UVA of

hydroxyphenyl-s-triazines type (HPT), which combines a high photo-permanence and a

high extinction coefficient with very low volatility, provides an outstanding color

stabilization potential for high-performance coatings (Schaller et al. 2008).

Although the weathering of wood has been investigated extensively, there is very

little information on the resistance of pressure-treated wood with coated clear acrylic

finishes containing an organic UV absorber and UV screener TiO2. Therefore, the

objective of this study was to evaluate the weathering behavior of treated samples of two



widely used species, namely European beech (Fagus sylvatica) and Scots pine (Pinus

sylvestris), finished with an acrylic-based organic UV absorber to get initial data and a

better understanding of their behavior.

EXPERIMENTAL Preparation Samples and Treatment Process

Defect-free samples with dimensions of 105 mm in length by 75 mm wide by 5

mm thick from the sapwood of two species, European beech (Fagus sylvatica) and Scotch

pine (Pinus sylvestris), were conditioned in a climate room having a temperature of 21 oC

and relative humidity of 65% until they reached a moisture content of about 12%. Later,

the samples were lightly sanded with 120 grit sandpaper.

Two types of water-based wood preservatives were used to treat the samples.

Micronized copper wood preservative (Celcure MC-850) is a water-based wood

preservative that contains a copper-based fungicide and an organic co-biocide (a

quaternary ammonium compound). It was supplied from the Osmose Company. Copper-

based wood preservative (Adolit KD 5) is a water-based wood preservative which

contains 20.53% copper (II) hydroxidecarbonate, 10.0% didecylpolyoxethylammonium

borate, and 8.0% boric acid. It was supplied from the Remmers Company. Four samples

from each species were treated with each type of chemical, employing a full-cell method

in a fabrication type tank. The treatment schedule consisted of an initial vacuum of 600

mm Hg for 30 min followed by a pressure of 12 bars for 60 min before samples were

removed from the tank. All of the specimens were lightly wiped to remove any excess

solution from their surface and weighed at an accuracy of 0.01g to determine the retention

value of the chemical in the samples based following equation (AWPA U1-09 2009).

3/10 mkgxV

GxCR

(1)

In Eq. 1, G is the amount of the treating solution absorbed by the sample based on the

initial and final weight of each block in grams; C is the preservative solution in 100 g of

the treating solution; and V is the volume of the sample. After the samples were treated

with chemicals, they were wrapped in plastic bags for one week at room temperature to

make sure that complete fixation of the chemicals was achieved. Table 1 displays

retention values of the samples treated with each type of chemical.

Table 1. Retention of Wood Preservatives

Wood Preservatives Retention (kg/m3)

Beech Pine

Adolit KD 5 (2%) 6.9 (0.68) 5.9 (0.19)

Celcure MC-850 (3%) 17.6 (0.29) 21.6 (0.49)

*Values in parentheses are standard deviations.



Coating and Artificial Weathering Processes of the Samples The water-based impregnation agent, having active ingredients of 1.20% propico-

nazol, and 0.30% iodopropynyl butylcarbamat, was used as a primer for the protection of

the samples against biological deterioration, including soft rot and blue stain. The primer

was applied to the samples at a spread of 120 g/m² using a brush. Two types of absorbers,

the UV screener TiO2 as an inorganic UV absorber and the UVA of hydroxyphenyl-s-

triazines class as an organic absorber, were used. Commercially produced finishing,

having acrylic resin, a copolymer dispersion of methylacrylate/methylmethacrylate/

butylacrylate, was used as a topcoat for the specimens. A small amount of defoamer and

2,2,4-trimethyl-1,3-pentandiolemonoisobutyrate, texanol as a coalescending agent was

added in the topcoat formulation to reduce the effect of other additives on the photo-

stabilization performance. Three layers of topcoats were also applied to each sample at a

spread rate of 100 g/m² by brush. Later, the specimens were sanded with a 240 grit size of

sandpaper and kept in room temperature for two days before applying the second layer of

topcoat. Table 2 displays the code of wood samples prepared for each variation.

Table 2. Wood Samples Prepared for each Variation Code Applied Methods Amount

Control Untreated control samples 4

UV1 Applied clear coating containing UV1* 4

UV2 Applied clear coating containing UV2* 4

AQ Treated with AQ* 4

MQ Treated with MQ* 4

MUV1 Applied clear coating containing UV1 after treated with MQ* 4

MUV2 Applied clear coating containing UV2 after treated with MQ* 4

AUV1 Applied clear coating containing UV1 after treated with AQ* 4

AUV2 Applied clear coating containing UV2 after treated with AQ* 4

*UV1: Tinuvin 477 DW, *UV2: Hombitec 402 WP, *AQ: Adolit KD 5, *MQ: Celcure MC-850

Formulation of the Coating Systems

These UV absorbers are synthesized by the Ciba Company (which is now part

of BASF) and the Sachtleben Company. Topcoats were formulated from the same

commercial acrylic resin, a poly-(methylacrylate/methylmethacrylate/butylacrylate)

copolymer dispersion. To exclude effects of other additives on the photo-

stabilization performance, only a small amount of defoamer and 2,2,4-trimethyl-1,3-

pentandiolemonoisobutyrate (texanol) as coalescending agent were used in the

formulations (see Table 3 and 4). Table 3. Ingredients of Clear Coat Containing UV Screener TiO2 Prepared

No Ingredient p.b.w* Type Supplier

1 Neocryl XK 90 73.7 Binder DSM NeoResins

2 Demi water 20.93

3 Texanol 0.67 Coalescent Eastman Chemical

4 Dehydran 1293 1.0 Defoamer Cognis

5 Coatex BR 100 P 0.6 Thickener Coatex

6 Rheolate FX 1070 1.3 Thickener Elementis Specialties

7 Inorganic UV Absorber-TiO2 1.8 UV Absorber Ciba

100.0

*Percent by weight



Table 4. Ingredients of Clear Coat Containing Organic UV Absorber Prepared

No Ingredient p.b.w* Type Supplier

1 Neocryl XK 90 73.7 Binder DSM NeoResins

2 Demi water 20.93

3 Texanol 0.67 Coalescent Eastman Chemical

4 Dehydran 1293 1.0 Defoamer Cognis

5 Coatex BR 100 P 0.6 Thickener Coatex

6 Rheolate FX 1070 1.3 Thickener Elementis Specialties

7 Organic UV Absorber-HPT

0.9 UV Absorber Sachtleben

100.0

*Percent by weight

Artificial weathering of the samples was performed in a QUV/spray accelerated

weathering test unit manufactured by Q-Panel Lab Products, Cleveland, USA. The unit is

equipped with UV-A lamps (W/m2 at 340 nm) maintaining the constant temperature of

60oC in the chamber during the test. The samples were exposed to weathering cycles of

UV-light irradiation for 2 hours followed by water spraying for 18 minutes (7 liters per

minute) for 28 days (ASTM G53-96 1988).

Color Measurement and Microscopic Analysis of the Samples Twenty color measurements of each sample were periodically carried out with a

Codec 400 Vis spectrophotometer manufactured by Phyma, Gaaden, Austria. The

reflection spectrum for each evaluation was developed by measuring a spot of 20 mm in

the 400 to 700 nm regions.

Color changes during weathering were constantly monitored at the same location

of the specimens based on the method of the Commission International de l‘Eclairage

(CIE) using color parameters, L*, lightness, a*, along the X axis red (+) to green (-), and

b* along the Y axis yellow (+) to blue (-). Color differences ΔE* of the samples were

evaluated using the software Primus 2.0, developed by Phyma, Gaaden, Austria and the

equation below,

ΔE* = (ΔL*2+Δa*

2+Δb*

2)1/2

(2)

Positive values of ∆b* indicated an increment of yellow color, while negative

values indicated an increase of blue color. Positive values of ∆a* resulted in a tendency

of the wood surface to be reddish, while negative values revealed a tendency to be

greenish. The negative ∆L* values probably occurred due to the fact that the wood

surface becomes rougher and darker during UV light irradiation. Measurements were

performed at the end of a UV irradiation step to achieve a consistent condition of the

samples during the color measurements (ISO 7724-2 1984).

Dry film thicknesses of the coated panel samples were also measured according to

ÖNORM B 3803 by employing a SZH Olympus light microscope. Ten measurements

were performed on the cross section of the samples to evaluate the film thickness of the

coating.



RESULTS AND DISCUSSION Color Change of the Samples

Table 5 displays the color changes of the European beech and Scots pine samples

in the form of CIELAB parameters, namely L*, a*, b*, and ∆E* after they were exposed

to 336 and 672 hours of weathering. It was found that ∆a* and ∆b* were reduced with an

increasing exposure time in an accelerated test cycle for the control samples from both

species.

The highest average value of ∆L* was determined for untreated samples. It was

also found that ∆L* was the most sensitive parameter to describe the surface quality of

the samples as results of irradiation and water spray. It appears that the chromaticity

coordinates, ∆a* and ∆b* were less influenced by the water spraying during the tests

(Temiz et al. 2005).

The values of ∆E* were substantially high as a result of 72 hours of exposure for

all types of the samples, suggesting that the color change of the samples took place during

the initial stage of weathering. Each species showed different color changes. However, in

general, softwood specimens had more rapid discoloration within the first 24 hours of

irradiation, while that was during the first 12 hours for the hardwood samples (Oltean et

al. 2008). It is a well known fact that the color change of the wood surface is not only a

visual defect but also reflects the chemical modification of the components in wood due

to photo-degradation. The color change is attributed to the carbonyl groups of conjugated

ketones, aldehydes, and quinines resulting from the modification of lignin and other

related compounds (Temiz et al. 2005; Pandey 2005). The highest average value of ∆E*

was found for untreated control samples for both species as a result of 336 h and 672 h of

weathering time. It appears that micronized copper and copper-based wood preservatives

containing copper treatments enhanced the color stabilization of the specimens compared

to that of the untreated control samples. The copper modifies the components in the cell

wall so that the wood surface can have higher resistance to color change, resulting in an

increased roughness of the wood surface. Wood ion complexes are also formed on the

wood surfaces, providing a possible higher resistance to the wood surface by blocking the

free phenolic groups, which are the reactive sites of photochemical reactions (Grelier et

al. 2000).

Findings in this work agree with those reported by previous studies stating that

copper-based treatment agents such as CCA, ACQ, DDAC, and ACC reduce the photo-

degradation rate by retarding the formation of carbonyl groups as a result of weathering

(Jin et al. 1991; Liu et al. 1994).

It was also reported that ACQ treatment retards the progressive photo-oxidation of

lignin compared to the photo-oxidation in untreated control samples (Jin et al. 1991).

Table 5 shows that during exposure of the acrylic clear-coats containing organic and

inorganic UV absorber, photostabilization performance was better than untreated control

samples and treated samples. The color changes, ∆E*, generally decreased with

increasing weathering times in accelerated test cycles for control of both species of

samples, except for clear-coats containing a UV absorber applied to samples.

The results of various studies showed that the clear-coat containing UV absorber

substantially prevented the color changes of the wood surface during weathering

(Williams and Brown 1993; Liu et al. 1994; Forsthuber and Grüll 2010). It also has been

reported that the acrylic resin provides a good performance, demonstrating that a resin



coat that contains a UV absorber is an adequate method for the protection of wood for

exterior applications (Decker et al. 2004; Custódio and Eusébio 2006). However, water-

borne coatings present the problem of the extractive bleeding in the case of pine because

of the presence of a resin that has water-soluble phenolic or polyphenolic compounds

(Dawson et al. 2008). The typical anatomical structure of softwoods and hardwoods

shows distinct differences. These structural differences of wood are the most important

factors that affect weathering characteristics (Williams 2005). Thus, the results of the

performance of a UV absorber on Scots pine sapwood photostabilization were different

than that of European beech sapwood samples, which do not have any extractives. In

Scots pine, the UV absorbers performed better than pretreatment with preservatives prior

to coating; the opposite trend was observed for beech.

The comparison of the efficiency of the difference in the UV screeners TiO2 and

the organic UV absorbers in the prevention of photo-induced discoloration of the coated

wooden substrate showed that TiO2 results in the least discoloration of the coated wood

panels (Forsthuber and Grüll 2010), as displayed in Table 5.

The negative lightness stability (∆L*) values occur during weathering because the

surface becomes darker, which is opposite of the positive lightness stability (∆L*) values.

In the early stages of weathering, dark woods tend to become light and light woods, dark.

Eventually, all woods become gray if fully exposed to UV light and water spray. As can

be seen in Table 5, ∆L* values of some samples were negative after 336 h exposure, but

values became positive after 672 h exposure.

The color of wood exposed outdoors is affected very rapidly. Generally, all

species exhibit discoloration toward yellow or brown shades due to the chemical

exposure, photo-oxidation of lignin, and wood extractives. This yellowing or browning

occurs after only several months of exposure in sunny, warm climates. In the absence of

micro-organisms, wood can weather to a soft silver gray color as a result of the leaching

of the decomposition products of lignin (Dawson et al. 2008). The influence of the

species on the member’s characteristics are related to the content of extractives, and

results of UV absorber performances on pine wood photostabilization was found to be

different than that from beech samples (Aloui et al. 2007). A comparison with the color

change results of pine samples showed good correlation between the chromospheres

formation and the discoloration of the surface of the samples. It seems that the recent

introduction of the lignin stabilizer concept would be one of the alternative ways to

overcome this problem. To verify the experience based on the findings that lignin

stabilization improved the color stability, the filter experiment was repeated in the

presence of a lignin stabilizer pretreatment (Schaller et al. 2008).

The color change parameters (∆L*, ∆a*, ∆b*, and ∆E*) of all the samples versus

the QUV exposure time are presented for Scots pine sapwood (Pinus sylvestris), as

illustrated in Fig. 1.

The color change rate of the surface of the samples due to QUV light exposure

was initially fast, and then the rate of change diminished to almost zero during the 168

hours of exposure. ∆E* varied from 0 to almost 10 units. Pine control samples had the

highest discoloration as a result of 672 h weathering with distinction from the other types

of samples.



Table 5. Color Change Values of Wood Surface after Artificial Weathering Test (672 hours)

Wood Species

Samples

Color Parameters

336 hours 672 hours

*∆L *∆a *∆b *∆E *∆L *∆a *∆b *∆E

Fag

us s

ylv

ati

ca

Control AQ MQ UV1 UV2 MU1 MU2 AU1 AU2

-16.9 (2.6) 10.6 (1.2) 16.2 (1.9) -2.0 (1.1) -1.4 (1.4) -3.5 (1.7) -3.8 (1.3) -4.5 (1.0) -5.2 (1.2)

-6.1 (0.9) 4.3 (0.8) 4.8 (1.0) -0.8 (1.3) -1.3 (0.6) 3.1 (0.5) 2.1 (0.7) 1.7 (1.0) 2.6 (1.1)

-9.7 (1.9) 6.1 (1.2) 5.4 (1.6) -0.2 (0.7) -1.0 (0.3) -1.7 (0.6) -2.5 (0.4) -3.2 (1.0) -0.6 (0.2)

20.4 (1.6) 13.0 (1.0) 17.7 (1.7) 2.0 (0.9) 1.3 (0.8) 5.0 (1.3) 5.0 (1.2) 5.8 (1.1) 5.8 (1.0)

-22.7 (1.3) 52.7 (2.1) 42.6 (2.0) 0.3 (0.9) 1.1 (0.7) -6.1 (1.2) -5.0 (1.6) -6.8 (1.3) -7.3 (1.1)

-7.8 (1.9) 2.6 (0.5) 3.7 (1.0) -1.9 (0.8) -2.8 (0.9) 3.8 (1.1) 3.2 (1.0) 1.9 (0.4) 3.3(1.2)

-12.2 (2.2) 7.5 (1.5) 10.1 (1.7) -0.6 (0.4) -1.3 (0.9) -3.4 (1.1) -3.3 (1.3) -5.2 (1.7) -1.9 (0.8)

26.9 (3.3) 12.5 (2.2) 16.6 (2.4) 2.0 (2.7) 1.9 (1.3) 8.1 (2.8) 8.8 (1.9) 8.8 (1.4) 8.3 (3.0)

Pin

us s

ylv

estr

is Control

AQ MQ UV1 UV2 MU1 MU2 AU1 AU2

-26.2 (3.9) 16.7 (2.8) 22.3 (3.1) -6.4 (1.7) -6.2 (1.3) 2.9 (1.1) 2.0 (0.8) 3.6 (1.5) 2.9 (1.0)

-3.0 (0.8) 7.5 (1.7) 6.3 (1.2) 5.7 (1.6) 5.0 (1.1) -4.0 (0.9) -2.2 (0.6) -5.2 (1.4) -4.1 (1.1)

-8.0 (2.3) 5.5 (2.1) 0.2 (0.8) -0.7 (0.9) -1.5 (1.1) 0.1 (0.9) 0.3 (0.7) 1.2 (0.9) 1.5 (1.1)

27.6 (2.1) 19.1 (2.7) 23.2 (1.6) 8.6 (1.2) 8.1 (1.6) 5.0 (1.0) 3.0 (1.7) 6.4 (2.4) 3.1 (1.1)

-22.1 (3.2) 57.1 (5.2) 48.5 (4.3) -8.0 (2.1) -7.0 (2.3) 2.8 (1.7) 1.9 (1.1) 3.6 (0.5) 2.8 (1.2)

-3.1 (1.9) 5.1 (3.2) 4.6 (2.6) 7.4 (2.1) 5.8 (1.8) -5.3 (1.1) -3.1 (1.4) -6.6 (0.6) -5.1 (1.5)

-8.8 (2.9) 14.6 (1.4) 13.7 (1.1) -0.6 (0.4) -0.8 (1.4) 0.9 (1.1) 1.6 (1.2) 1.1 (0.8) 0.1 (0.7)

24.9 (3.2) 22.1 (2.1) 23.9 (3.9) 10.9 (2.1) 9.1 (1.1) 6.0 (1.6) 3.9 (1.3) 7.5 (2.1) 5.9 (1.6)



Fig. 1. Color change of weathered Scots pine (Pinus sylvestris) samples

The highest color change (∆E*) was determined for the samples treated with

micronized copper and copper-based preservatives, while the lowest color change (∆E*)

was found for samples treated with wood-coating containing UV absorbers. When

comparing the efficiency of the different UV absorbers and treatments in the prevention

of the photo-induced discoloration of the coated wood, clear-coats containing a UV

absorber applied after treatment with micronized copper and copper-based wood

preservatives resulted in the least discoloration for pine samples, as illustrated in Fig. 1.

Fig. 2. Color change of weathered European beech (Fagus sylvatica) samples



The color change parameters (∆L*, ∆a*, ∆b*, and ∆E*) of European beech

(Fagus sylvatica) sapwood samples versus the QUV exposure are illustrated in Fig. 2.

The specimens treated with micronized copper and copper-based wood preservatives for

672 h of weathering somehow had a similar color to those of the control samples.

However, the ∆E of the treated samples was lower than that of the control samples. The

lowest color change (∆E*) was determined for the untreated wood-coated having UV

absorbers. It was observed that during the 672 h UV exposure, color changes showed a

progressive increase when powerful UV absorbers, such as an organic UV absorber and

UV screener TiO2, were used. On the other hand, for the control samples and those

finished without using the UV absorber, the samples showed an increased color change

during 336 h exposure time.

Coating Penetration of the Samples

Generally penetration of preservatives in Scots pine (softwood) is expected to be

higher than beech (a hardwood having vessel elements). But the depth of the coating

penetration was greater in beech than pine according to the results of this study. The

extractives in pine wood and swelling of the cell wall due to solvent ingress have an

important influence on penetration (Bulcke et al. 2008). Based on the microscopic

evaluation, it was observed that the treated pine samples had a higher coating penetration

that those of untreated samples. However, this was exactly opposite in the case of beech

samples. Moreover, treatment with wood preservatives did not significantly affect the

drying rates of the finishes on the wood when compared to the untreated wood samples.

The color change of the untreated pine wood was higher than that of the treated pine

samples. This could be due to lower penetration of the coating chemical in the untreated

pine specimens. The coating did not penetrate deeply into the pine samples because

penetration was confined to the exposed lumens of the tracheids in the outermost cell

layer, as shown in Fig. 3. However, the ray cells exhibited deep penetration. In

comparison, in both beech samples coated with UV screeners TiO2 and organic UV

absorbers, the ray cells had deeper penetration, as illustrated in Figs. 3 and 4.

Fig. 3. Light micrograph of transverse section through beech and pine wood applied clear-coats containing UV screeners TiO2



Fig. 4. Light micrograph of transverse section through beech and pine wood applied clear-coats containing organic UV absorber

The depth of the coating penetration was similar for both beech samples to which

clear-coats containing UV screeners TiO2 and organic UV absorbers were applied. The

depth of the penetration in beech woods was more uniform and confined to 2 to 3 outer

cell layers. The rays penetrated well in all samples, including both pine samples having

clear-coats, UV screeners TiO2, and organic UV absorbers. It is apparent from the

micrographs that the depth of the coating penetration was greater in the beech samples as

compared to that of the pine specimens. The reason for the differences of the depth of the

penetration in pine compared to beech could be due to the anatomical structure of the

species.

The coating thickness was reduced when the pine wood was pretreated with

increasing amounts of micronized copper and copper-based preservatives. However,

during sectioning, the coating tended to detach in places from the surface of the

pretreated beech samples, suggesting that despite increased porosity, the coating did not

interlock into the surface sufficiently for the wood-coating interface to overcome the

mechanical stresses generated during sectioning.

The tracheids in the surface layers of the pretreated beech woods tended to

separate in the region of the middle lamella, and the cells of this region would be one

distinct point of weakness if the coating did not penetrate sufficiently and uniformly

enough to stabilize this region (Dawson et al. 2008).

The wood surfaces exposed to UV irradiation and water spray had several checks,

splits, and cracks. Visual evaluation was carried out to evaluate the cracks caused by

weathering. The system used to quantify the size of the cracks is described in ISO 4628.

The coated sample showed no cracks, but had basic changes due to weathering. However,

surface cracks were found on both of the control samples and treated samples. Pine

control samples had the highest numbers of cracks identified as class 5, while beech

control samples were within class 3. The treated samples had no big cracks and

substantial modification on their surface was observed as class 2.



CONCLUSIONS

1. The most effective protection for stabilizing wood color involved pretreated wood

coated with a UV absorber in the case of pine, and untreated wood coated with a UV

absorber in the case of beech.

2. The results showed that the influence of the coating containing UV screeners TiO2

and organic UV absorber (UVA of hydroxyphenyl-s-triazines type) is similar with small

differences.

3. The coating penetration increased with higher coating absorbency for pretreated

surfaces for pine. The organic UV absorber represents an outstanding stabilization

potential for high-performance coatings.

4. Other methods provided better protection against color changes relative to untreated

control samples. As some cracks were observed on the surface of the control samples and

treated wood, UV absorbers prevented the cracking of the clear-coat surfaces during

weathering.

ACKNOWLEDGMENTS

We would like to thank Holzforschung, Austria for having provided us with

research material, and University of Natural Resources and Applied Life Sciences

(BOKU), Institute of Wood Science and Technology for laboratory support.

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